How do I use this?

Mempool and TX Fee Calculator

Plot your transaction in the mempool and estimate how much you need to spend in Bitcoin transaction fees.

Address Format:
If you have multiple formats, select the most common input address format.
P2PKH (Pay To Pubkey Hash)
Legacy address format.
Address starting with 1...
Transaction size
Transaction fees
SegWit Fee Savings upgrade to segwit to save on fees.

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How bitcoin transactions work? What is the mempool?

When you send a bitcoin transaction, it is broadcasted to all nodes in the bitcoin network. And then it goes to the mempool. The mempool is a collection of unconfirmed transactions waiting to get into the next blocks. A transaction is only confirmed when it is registered in a block.

Why is transaction size important?

A block has a limited block size and it cannot fit all transactions in the mempool at once. To get into a block, you will need to pay a fee per each vbyte of your transaction, also know as fee rate. Fee rate is measured in satoshis per vbyte, or just sat/vbyte.

Miners usually prioritize transactions with a high fee rate to get in the next block. Thus, transactions form a queue in the mempool waiting to be confirmed, so that transactions that pay more per vbyte stay ahead.

What is this chart?

The chart above represents the transactions in the mempool sorted by their fee rate.
Transactions paying more fee per vbyte are on the top and will get into the next blocks sooner than transactions on the bottom. The black lines mark the virtual size of the next three blocks. As more transactions are always being broadcasted to the mempool, a feerate inside the next block limit does not guarantee that it will be confirmed soon, but it is a good estimate.

How is transaction size calculated?

Every input and output has a size, and that size depends on its respective address format.

This tool calculates the size of bitcoin transactions (in vbytes). You just need to enter the number of inputs and outputs, your address format, and choose a fee per byte.
Your transaction size and cost are displayed in BTC and USD value. Compare the value with the size of the next two blocks (marked in the chart) and estimate the transaction fee you want to pay.

How do I pay less in fees?

There are 3 things that control a transaction fee:

  • Number of inputs and outputs.
  • Address Format. Use segwit to pay lower fees, specially P2WPKH and P2WSH address formats.
  • Fee per vybte. Usually measured in satoshi per vbyte (sat/vbyte)
You need to make your transaction as small as possible. Reducing the number of inputs and outputs will help. Use segwit, especially Bech32 address formats and choose the appropriate fee rate (if you are not in hurry, use a low fee/vbyte).

Further Reading and References:

Weight Units in Bitcoin Wiki.
Segregated Witness (SegWit) in Bitcoin Wiki.

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